page validator: additional checks

[bertsky]

Beyond actual (syntactic) schema violations ("validity") and conventional (semantic) problems ("inconsistency"), we might want to check for and repair additional issues:

• if /PcGts/Page/ReadingOrder or any of its children is empty (in which case PageViewer fails to load) – as repaired by https://github.com/bertsky/workflow-configuration
• if any /PcGts/Page/ReadingOrder//@regionRef does not point to an existing segment identifier (in which case PageViewer fails to load) – as repaired by https://github.com/bertsky/workflow-configuration
• if any //TextEquiv contains neither PlainText nor Unicode (in which case PageViewer fails to load) – as repaired by https://github.com/bertsky/workflow-configuration
• ...?

[bertsky]

Also:

• if any /PcGts/Page/ReadingOrder/(OrderedGroup|OrderedGroupIndexed)/@index is not in order (or clashing)
• if any //TextEquiv/@index is not in order (or clashing)